For several reasons, an embryo is less likely to survive when placed in the uterus at this younger age. For example, the embryo's own genes do not take control until four cells have developed; before that, genetic messages come from the egg. Therefore, at three days, 20% to 30% of embryos which appear normal under the microscope are actually genetically imbalanced and will not survive after implantation. Therefore more embryos are generally implanted than are needed in the hope that one will survive. Unfortunately, this results in high multiple pregnancy rates; usually 25%—40% of those who conceive have multiple pregnancies, with triplets not unusual.

As our laboratories become better able to support embryo growth, we can hold an embryo in the lab until day 5-6. This improves results in two ways. First, because genetic abnormalities are much easier to detect by the fifth day, we end up with a population of embryos more likely to be genetically normal and capable of ongoing development. Second, we therefore need fewer embryos per implantation, reducing the multiple pregnancy rate while maintaining high pregnancy rates.

Sometimes the embryo needs additional help in the implantation process. Normally, when the embryo reaches the uterus it must break through its own outer membrane, called the zona pellucida, in order to implant. This is known as "hatching" (imagine a chicken hatching from an egg). When the embryo develops in the laboratory, the outer membrane may become thicker and harder than under normal conditions. This can impede the ability of the embryo to break through its wall and implant in the uterus. Sometimes we aid the embryo by purposefully weakening the membrane, either applying an acidic solution at one point, or making a small hole with a tiny glass needle or special laser (still experimental). This is called "assisted hatching." Authorities in the field do not universally accept assisted hatching as a truly beneficial aid, but some feel that it may be helpful, especially in women over 35.

Another technique used in IVF is "co-culture." Co-culture refers to adding live cells grown in tissue culture from the tube, uterus or sometimes kidney of human, primate, or bovine (cow) sources in order to supply hormones, growth factors and nutrients to the embryo while in the incubator. This approach is far from mainstream, and will probably be used less often as media continue to improve.

Phase 4: Phase four is the actual embryo replacement. This is usually quite straightforward. The embryo(s) are drawn into a soft plastic catheter, which is placed through the cervix into the uterus, sometimes with ultrasound guidance, so that the embryo can be delivered to its natural home in the uterus. No anesthesia is necessary. Hormonal values may be checked over the next week, and frequently progesterone supplements are given as injections, vaginal suppositories, vaginal gels, and less commonly by mouth. A pregnancy test usually is done 12-14 days after retrieval (Phase 2).

Variants of IVF

IVF technology has allowed development of egg donation programs whereby women with poor or absent ovarian function can become pregnant using eggs from a healthy young woman. Women who have ovarian function, but who lack a uterus or who have a uterus that is reproductively incompetent, may supply eggs to be fertilized and placed in a recipient's uterus. Known as "carrier gestation," this type of pregnancy is initiated with husband and wife both having genetic input. This is different from surrogate parenting, in which a surrogate mother is inseminated with the sperm from the husband of the patient so that the surrogate mother supplies the eggs.

Frozen-thawed replacement

Sometimes, dependent on egg quantity, quality, and sperm function, we have more embryos than can reasonably be returned to the uterus. These embryos can be frozen (cryopreserved) in liquid nitrogen, probably for an indefinite period of time. In actual practice, if pregnancy has not occurred from the “fresh” transfer of embryos, the stored embryos can be thawed for replacement in the next cycle without the need for stimulation and all the hormonal monitoring. This is referred to as "frozen-thawed replacement."

Additionally, a couple who wants another child can return for frozen-thawed replacement several years after a successful delivery from previous IVF. Studies have shown that the freeze-thaw process does not introduce any increased risk of malformation in the offspring. Embryos can be frozen and thawed with fair success, and sperm has been stored for years and used later. Storage of eggs, however, is quite another matter. We are still in the early learning phases of how to successfully accomplish this. Banking of eggs would allow women facing chemotherapy that might permanently destroy ovarian function to have a reproductive option at a later date, and would also allow women to initiate pregnancy later in life when career and development goals have been satisfied.

Intracytoplasmic sperm injection (ICSI)

Some men have a low enough sperm quality such that standard IVF is not sufficient to induce pregnancy. Intracytoplasmic sperm injection (ICSI) is an IVF technique that involves drilling a small hole through the outer membranes of the egg (oocyte) and introducing a single sperm into the interior (cytoplasm) of the egg with a hollow glass needle. This technique overcomes most of the sperm abnormalities that prevent normal fertilization.